The present invention relates to an amphibious vehicle having a decoupler for coupling/decoupling a drive shaft of the vehicle, and more particularly to an amphibious vehicle having a drive-shaft decoupler for engaging and disengaging a drive shaft which driveably connects an output from a power train of the vehicle with the wheels or marine propulsion system of the vehicle.
It has been found convenient to drive the marine propulsion systems of an amphibious vehicle through the transmission by which the wheels are also driven. With this arrangement it is necessary to disengage the drive to the wheels while the drive to the marine propulsion is engaged when the amphibious vehicle is in water mode. It is also desirable to be able to decouple and couple the drive to the wheels and marine propulsion system independently of one another as the vehicle makes the transition between land and marine modes of travel.
In the case of a transmission which incorporates a final-drive and differential unit as an integrated part of the whole transmission, it would only be possible to incorporate a decoupler into the transmission by designing a new internal arrangement. For amphibious vehicles, which are used in specialized applications and are produced in relatively low volumes, a dedicated transmission would be prohibitively expensive.
Therefore, on an amphibious vehicle in which the engine drives the road wheels and the marine propulsion system through an integrated transmission/differential, it has been found necessary to provide external drive shaft decouplers to disconnect the drive between the differential and the driven wheels, and between the transmission and the marine propulsion system. Typically a decoupler is provided in the driveline between the transmission and each driven wheel. However, it is possible to use a decoupler in the driveline between the transmission and only one of the driven wheels, since disconnecting drive to one of the driven wheels will effectively disengage drive to both wheels due to the effects of the differential.
A problem with known external drive shaft decouplers is the amount of extra space they require. This is a particular problem in amphibious vehicles in which the wheels are designed to retract upwardly and inboard of the vehicle for use of the vehicle in water. In such vehicles the provision of wheel retraction systems and specialized suspension systems reduces the available space for external drive shaft decouplers.
A further problem which arises is the need to synchronise the speeds of the input and output means of the decoupler when drive is being coupled. This problem arises, for example, when the vehicle is preparing to leave the water with the wheels deployed. In these circumstances it is necessary for drive to be maintained to the marine propulsion system, in order to push the vehicle towards the shore, whilst drive to the road wheels is coupled. This enables the vehicle to propel itself out of the water using a combination of drive from the marine propulsion system and the road wheels. It is necessary, therefore, for the decoupler to have a clutch means to progressively engage the drive between the differential output of the transmission (which may be spinning at 1000 RPM), and a rotating assembly consisting of the drive shaft, brake disc, hub and wheel (which initially will be stationary in the water) representing the inertia to be overcome.
A similar problem arises when the vehicle enters the water, when it is desirable to couple drive to the marine propulsion system whilst drive to the road wheels is maintained. This enables a smooth transition from land to waterborne use of the vehicle but requires a stationary marine propulsion system to be coupled to a rotating power take-off shaft of the transmission.
Furthermore, where the decoupler is to be used in the driveline between the transmission and a drive shaft for a wheel of an amphibious vehicle, the decoupler must be capable of handling the high torque loads which are experienced by the drive shaft. For example very high torque loads are experience in such drive shafts when drive to the wheels is engaged, wherein engine torque (say 250 Nm) is multiplied by a first gear ratio (say 4:1) times a final drive ratio (say 3.5:1):250×4×3.5=3500 Nm
When shock-loads from wheel/ground-contact torque reactions are factored in, it is common to allow for 10-12,000 Nm peak torque loads for the drive shafts of an average sized road vehicle.
It is an object of the invention to provide an amphibious vehicle having a decoupler which is capable of meeting the above requirements and which requires less space than known decouplers.